An Exploration of Common Greenhouse Gas Emissions by the Cyanobiont of the Azolla–Nostoc Symbiosis and Clues as to Nod Factors in Cyanobacteria

Abstract

Azolla is a genus of aquatic ferns that engages in a unique symbiosis with a cyanobiont that is resistant to cultivation. Azolla spp. are earmarked as a possible candidate to mitigate greenhouse gases, in particular, carbon dioxide. That opinion is underlined here in this paper to show the broader impact of Azolla spp. on greenhouse gas mitigation by revealing the enzyme catalogue in the Nostoc cyanobiont to be a poor contributor to climate change. First, regarding carbon assimilation, it was inferred that the carboxylation activity of the Rubisco enzyme of Azolla plants is able to quench carbon dioxide on par with other C3 plants and fellow aquatic free-floating macrophytes, with the cyanobiont contributing on average ~18% of the carboxylation load. Additionally, the author demonstrates here, using bioinformatics and past literature, that the Nostoc cyanobiont of Azolla does not contain nitric oxide reductase, a key enzyme that emanates nitrous oxide. In fact, all Nostoc species, both symbiotic and nonsymbiotic, are deficient in nitric oxide reductases. Furthermore, the Azolla cyanobiont is negative for methanogenic enzymes that use coenzyme conjugates to emit methane. With the absence of nitrous oxide and methane release, and the potential ability to convert ambient nitrous oxide into nitrogen gas, it is safe to say that the Azolla cyanobiont has a myriad of features that are poor contributors to climate change, which on top of carbon dioxide quenching by the Calvin cycle in Azolla plants, makes it an efficient holistic candidate to be developed as a force for climate change mitigation, especially in irrigated urea-fed rice fields. The author also shows that Nostoc cyanobionts are theoretically capable of Nod factor synthesis, similar to Rhizobia and some Frankia species, which is a new horizon to explore in the future.

A Hypothesis on How the Azolla Symbiosis Mitigates Nitrous Oxide Based on In Silico Analyses

“…WP_013192178.1 was earlier identified by the first author of this study, using sequence comparison to a nosZ-containing cyanobacterial protein [3]. The comparison between WP_013192178.1 and the nosZ-containing protein yielded ~31.43% identity and ~55% sequence coverage [3]. Note that the assignment of nomenclature was based on computational methods and not wet experiments.…”

“…Before the oxygenation of the ancient world billions of years ago, prokaryotic lifeforms had to contend with nitrous oxide that saw its birth from corona discharge and not as lightning strikes-although there is literature attributing nitrous oxide emanation to streaks of lightning-which anucleated microorganisms reduced using an enzyme designated as nitrous oxide reductase [1]. Nitrous oxide is a greenhouse gas that subsists in the atmosphere for 114 years and is 298-fold more potent than the benchmark gas of carbon dioxide in global warming potential [2,3]. Nitrous oxide reductases are known to be of the same protein fold as cytochrome oxidases (subunit II), the former being the more pre-historical one and the latter evolving from the ancient fossil of a fold, although this is now disputed [4].…”

Early events of the endophytic symbiotic betweenOryza sativaandNostoc punctiformeinvolve the SYM pathway

“…Reinforcing our results, it has to be noted that Nostoc genes with homology to nodE had been already detected by Southern blot in previous studies (Rasmussen et al, 1996). In silico studies in Nostoc have recently identified two ORFs encoding putative homologs of Rhizobium NodB and NodC proteins (Gunawardana, 2019). Rhizobial NFs are induced by flavonoids derived from prospective plant hosts (Mus et al, 2016).…”

An in silico Study of Two Transcription Factors Controlling Diazotrophic Fates of the Azolla Major Cyanobiont Trichormus azollae

“…A newer paradigm is presented by Azolla due to the plant's potential as a voracious carbon sink-the Rubisco Carboxylase is the most common enzyme in plants -, which can be utilized 2 Bioinformatics and Biology Insights in abundance due to the strong nitrogen fixation performed by the cyanobiont. 7 I showed in an earlier publication that Trichormus-Azolla symbiosis is a sound future bio-technological ally in a prospective "field-based" setting, due to the absence of nitrous oxide and methane emitting enzymes in the cyanobiont's proteome. 7 A third benefit that can be reaped from Azolla, is its potent ability to quench harmful heavy metals such as lead, chromium and to a lesser extent cadmium.…”

“… 7 I showed in an earlier publication that Trichormus-Azolla symbiosis is a sound future bio-technological ally in a prospective “field-based” setting, due to the absence of nitrous oxide and methane emitting enzymes in the cyanobiont’s proteome. 7 A third benefit that can be reaped from Azolla, is its potent ability to quench harmful heavy metals such as lead, chromium and to a lesser extent cadmium. 8 Again, phytoremediation is dependent on proteins such as “sequestering” metallothioneins, and therefore, nitrogen fixation by the cyanobiont, once again, is of strong importance.…”